Memory devices contain memory cells that store data and can be read to retrieve the stored data. For various reasons, occasionally some memory cells are not able to reliably perform their storage or readout functions. Such memory cells may become weak and may lose their contents over time or they may become bad and cannot be either erased, re-programmed or both. The capacity for reliable memory storage in memory devices made from degradable memory cells may decrease with use. The degradation in a memory device's capacity may be exacerbated by strategies that discard memory elements that are accessed more than a predetermined number of times.
Memory cells may be addressed individually or may be arranged in blocks of a predefined size, such as 2 Kb, and addressed by block. When memory cells are arranged in an array, they may be addressed by cell row and cell column. Alternatively, when memory cells are organized into an array of blocks, they may be addressed by block row and block column.
Generally, memory devices are described by their memory capacity or size. Size descriptors typically give a memory device's capacity in either megabytes (Mb) or gigabytes (Gb). Smaller storage units, such as blocks, may be given in kilobytes (Kb). To define these terms precisely, a byte is 8 bits, a kilobyte is precisely 1024 bytes, a megabyte is 1024 Kb (1,048,576 bytes), and a gigabyte is 1,073,741,824 bytes. A Kb, Mb, and Gb are 210, 220, 230 bytes, respectively.
The reliability of a memory device containing numerous memory cells can be a function of the memory cell semiconductor device technology, the semiconductor material system, the memory cell semiconductor process technologies, the likelihood and severity of various failure modes, and the failure criteria. Examples of semiconductor material systems include silicon, silicon-germanium, indium phosphide, gallium arsenide, and other compound semiconductors. Examples of semiconductor device technologies include complementary metal oxide semiconductor (CMOS, a silicon technology), bipolar junction transistor (BJT), integrated injection logic (I2L), and others.